1. Field of the Invention
The present invention relates in general to a noise insulating member having a low degree of water absorbency and a low degree of oil absorbency. More particularly, the invention is concerned with a noise insulating material which is made of a polyurethane foam having a low degree of water absorbency and a low degree of oil absorbency, so as to be suitable for forming a noise insulating member for use in an automotive vehicle or a hydro-bike.
2. Discussion of the Related Art
There have been conventionally used various types of noise insulating member in an automotive vehicle, for reducing undesirable transmission of noise into the passenger compartment or to the outside of the vehicle. In the engine room of the vehicle, for example, an engine topcover 2, an engine sidecover 4, an oilpan cover 6, an engine undercover 8 and other noise insulating covers are provided around the engine, i.e., a generator of a noise, as shown in FIG. 1, in the interest of reducing radiation of the noise from the engine. Each of these noise insulating covers consists of a metallic plate or other rigid member, and a rubber or urethane foam or other foam body which is fixed to the rigid member. In addition to these noise insulating covers, a spacer 10 formed of a rubber or urethane foam or other foam body is positioned in a space between the engine and accessories such as a fuel injection pump, which are located adjacent to the engine, in the interest of minimizing generation of standing wave in the space. Further, in the engine room of the vehicle, there are provided a hood silencer 12, an intake pipe silencer (not shown), a dashboard silencer (not shown) and other noise insulating covers or members which are formed of respective foam bodies.
The above-described noise insulating members provided in an automotive vehicle are likely to be exposed to water or raindrop, in some cases, for example, where the vehicle runs in a rain, or where the vehicle is washed or cleaned with water in a periodic inspection or on other occasion. In this view, the foam body used to constitute each of these noise insulating members is required to have a high degree of water repellency or a low degree of water absorbency, so that the water or raindrop is not absorbed into the foam body even where the noise insulating member is exposed to the water or raindrop. Further, each of the noise insulating members 2, 4, 6, 8, 10, 12, which are located around the engine, needs to be constituted by the foam body having a high degree of oil repellency or a low degree of oil absorbency as well as a high degree of water repellency or a low degree of water absorbency, since these members 2, 4, 6, 8, 10, 12 are possibly exposed to an engine oil or other lubricant which has leaked from the engine. It is important to note that the noise insulating member may become inflammable if the member contains the oil absorbed therein.
In recent years, there is an increasing number of people who enjoy riding on a water- or hydro-bike as an aquatic sport. For the purpose of reducing radiation of noise from the engine in such a hydro-bike, it has been proposed to attach a suitable noise insulating member to an inside wall of a lateral part of the main body or hull, or to other part of the hull. A reference numeral 14 in FIG. 2 designates such a noise insulating member, by way of example, which is provided for the above purpose. Like the above-described noise insulating members 2, 4, 6, 8, 10, 12 which are employed in a road vehicle, this noise insulating member 14 needs to have not only a high degree of water repellency but also a high degree of oil repellency, because the hydro-bike runs on water, and the noise insulating member 14 is positioned near the engine.
The noise insulating member for use in an automotive vehicle or a hydro-bike is constituted by a rubber foam as the foam body. Where the noise insulating member serves as the above-described spacer 10, the noise insulating member is required to simply reduce a resonance noise, and to have a high degree of heat resistance rather than a high degree of acoustic absorption coefficient. To this end, the spacer 10 is constituted by a rubber foam having a closed-cell network which is made of CR, EPDM or other material, and in which there is a predominance of non-communicating cells. However, the rubber foam made of EPDM suffers from a low degree of oil resistance. In addition, it is difficult to obtain a soft foam body having a density of as small as 0.1 g/cm.sup.3 or so, by using a rubber composition, since the rubber foam has a weight larger than the urethane foam or other foam body.
The urethane foam (polyurethane foam) is also used as the foam body for giving the noise insulating member. The urethane foam provides a soft foam body having a density of as small as 0.1 g/cm.sup.3 or so. Thus, the urethane foam is advantageously used where the foam body is required to have a light weight. In an attempt to improve waterproof property of the urethane foam, it is proposed to cover the surface of the urethane foam body with a film or other coating, as disclosed in JP-A-08-40154. Further, it is proposed to use a foam body is which is obtained by using as additives a hydrophobic polyol and an asphalt component, as disclosed in JP-B-57-22051 or JP-B-61-50965. However, these disclosed arrangements do not provide a substantial improvement in the water repellency or oil repellency of the foam body, but rather lead to an undesirable increase of the weight of the foam body and also a further complicated operation in the production of the noise insulating member, deteriorating the productivity.
It might be possible to improve the waterproof property of the foam body, by increasing a degree of independency of the cells of each other, or by providing the structure of the foam body with a certain degree of hydrophobicity. The degree of independency of the cells could be increased by reducing the size of each cell, namely, by increasing the density of the foam body, or alternatively by closing the cells from each other. However, these methods cause an increase of the weight of the noise insulating member and also deteriorate the appearance of the foam body due to shrinkage of the material in the forming process.